The invention relates to polyurethane elastomers with specific structural units, a method for their production as well as their use in melt-spin and extrusion processes for the production of films or fibers.
Polyurethane elastomers are block copolymers built of regularly arranged soft and hard segments. The soft segments comprise long, randomly arranged and flexible chains which lend to the fibers the requisite rubber-like elasticity. The properties can be varied with the relative molar mass and the type of soft segment with respect to elongation and force of elongation. The hard segments are responsible for the restoration of the molecule chains after the deformation. They comprise short-chain crystalline regions. As fixed points, the primary task of the hard segments is preventing the polymer chain from sliding off under the effect of mechanical forces. After a deformative extension the restoring forces present in the elastomer cause a contraction almost to the starting length. The remaining difference in length is referred to as residual elongation.
In general, the polyurethane elastomers are obtained according to a two-stage process in which in a first reaction step higher molecular diols are converted with diisocyanates to prepolymers, which, in a second step, react with so-called chain lengthening means to form high-molecular products. In the first reaction step excess quantities of diisocyanate are used so that the prepolymer is terminated at both ends by an isocyanate group. The chain lengthening means are bifunctional, low-molecular compounds with terminal reactive hydrogen atoms, most often dihydroxy or diamine compounds. They react with the prepolymers to form the corresponding carbamic acid derivatives, i.e. the polyurethane elastomers, respectively polyurea urethane elastomers. In the macromolecule chains the soft segments formed of the higher-molecular diols alternate with the rigid hard segments formed by the reaction of the chain extenders with terminal isocyanate groups.
The different chemical compositions of hard and soft segments as well as their different polarities and molecular weights leads to demixing. Hydrogen bridge bonds between adjacent chains lead to the fact that the hard segments become juxtaposed in parallel. The long mobile molecule chains in between form entanglements and loops which during elongation of the wide-mesh network become detached and elongated. The interaction between hard segments prevents the plastic flow of the molecule chains in the elongated state. The elongation of the macromolecules is tied to a transition into a higher-ordered conformation and a decrease of entropy. Due to the thermal motion of the molecules, they return to the state associated with higher entropy of the looping after relief of loading. Under strong mechanical loading, however, a permanent elongation can occur so that the polymer chains slide off in the elongated state.
The hard segments in polyurethane elastomers have an elongation-limiting cross linking function as well as also a decisive influence on the thermal behavior, respectively the melting range. The urea groups formed when using diarnine chain extenders are more polar than the urethane groupings formed through glycol extension. The stronger hydrogen bridges in the polyurethane elastomers lead to greater demixing occurrences of hard and soft segments, and thus better elastic properties as well as also higher hard segment softening points.
From the literature various spinning methods for the production of elastomer yarns are known. The major portion of the elastomer fibers is produced according to the dry-spin method. Here a highly viscous solution is pressed through multihole nozzles in perpendicularly disposed heated spinning shafts. Simultaneously, hot air is supplied in order to evaporate the solvent and to solidify the filaments. The spinning rate is in the range from approximately 200 to 600 m/min. Due to the low dwelling time in the spinning shaft, and in order to ensure complete solvent removal, the single titers are limited to a maximum of 20 dtex. The production of the end titers takes place by joining corresponding single capillaries with the application of a false torque.
In wet-spinning the prepared polymer solutions are spun into a coagulation bath. The yarns are subsequently washed, bonded one to other and dried. The draw-off rate is approximately 100 m/min.
The reactive spinning method combines a chemical reaction with the spinning process. The prepolymer is extruded through multiple hole nozzles into a spinning bath of, for example, aliphatic diamines. On the surface of the filament an immediate isocyanate-amine reaction to polyurea urethanes take place. The relatively solid skin permits the secure spinning process. The interior of the fiber is cured through treatment with hot water or through the reaction with diamine alcohol or toluene. The spinning rates are in the samerange as in the wet-spinning process.
In melt-spinning the polymer is melted in a cylinder and the melt is pressed through the die plate with gear pumps or extruder worms, the exiting fibers solidify in the air. The advantage of this technology lies in the solvent-free spinning into yarn. This economically significant method has until now not been applied to polyurethane elastomers lengthened with amino-containing chain extenders. These polymers usually decompose before melting due to the high softening point of the hard segments.
Commercially available melt-spun elastomeric polyurethane fibers therefore are based on hydroxyl group-containing chain extenders. F. Foume (Chemiefasern/Textilind. 96 (1994), 392-398) reports about the Japanese manufacturer Kanebo who operates pilot plants in which the polyurethane-ester fiber xe2x80x9cLobellxe2x80x9d is obtained according to the melt-spin method. The melt-spun polyether-ester yarns xe2x80x9cRexexe2x80x9d and xe2x80x9cSpantelxe2x80x9d of the companies Teijin, respectively Kuraray Co., have been available on the market since 1993. The mechanical properties of these fibers are not satisfactory because here in the hard segments urethane groups are present instead of urea groups which, as explained above, lead to lower mechanical stability due to weaker hydrogen bridge bonds.
An improvement of the mechanical textile properties was attained through covalent cross linking of the hard segments (F. Hermanutz, P. Hirt, Chemiefasern/Textilind. 96 (1994), 388-391). By using double bond-containing chain extenders, centers were created for this purpose which are capable of being cross linked. Through electron or UV radiation subsequent polymerization can be triggered after the spinning. These polyurethane elastomers, however, are subject to the restriction that the diamine chain-extending polyurethane elastomers are not melt-spinnable. These known polyurethane elastomers, furthermore, exhibit strong yellowing after exposure to electron or UV radiation.
It is therefore the object of the present invention to make available polyurethane elastomers which, due to suitable melting points, can advantageously be melt-spun and, optionally following suitable secondary treatment, for example, by radiation with high-energy radiation, lead to fibers with improved mechanical properties.
According to the invention this object is solved through a polyurethane elastomer which comprises structural units of the following type: 
wherein:
the groupingxe2x80x94Oxe2x80x94R1xe2x80x94O represents a macrodiol group of a molecular weight from approximately 500 to 10000,
R2 a bivalent aliphatic, cycloaliphatic, and/or aliphatic-cycloaliphatic group; and X a short-chain, olefinically unsaturated group,
Y NH or O as
n an integer from 1 to 10, and
m an integer of at least 4.
In a polyurethane elastomer accordingly the moieties R1, R2 and X are of significance.
The moiety R1 can be traced back to a macrodiol. Preferably linear diols are substantially used which, in addition to the terminal hydroxyl groups, carry no further groups reacting with isocyanates. The macrodiols have a molecular weight of approximately 500 to 10000, preferably approximately 700 to 5000, in particular approximately 1000 to 3000. The molecular weight is to be understood as weight-averaged mean molecular weight. If the macrodiol moieties are too short, the cohesion energy difference between hard and soft segments becomes less which leads to stronger phase mixing and thus poorer elastic properties. Macrodiols with a low second order transition temperature are preferably used.
In general, the second order transition temperature of the macrodiols applied is approximately xe2x88x9235xc2x0 C. to xe2x88x9260xc2x0 C. Polyester, and polyether glycols, are preferably used. As polyether glycols are denoted polyethers with terminal hydroxyl groups. Polyalkylene glycols are preferably used. Preferred examples are polyethylene glycol, polypropylene glycol and/or polytetramethylene glycol, of which the latter is especially preferred. Polytetramethylene glycol is also denoted as polytetrahydrodfuran and can be produced through ionic polymerization of tetrahydrofuran with acid catalysts. Suitable copolymers are obtained through copolymerization from tetrahydrofuran with propylene oxide, ethylene oxide and glycols. Elastomers synthesized from polyether glycols are distinguished through advantageous low-temperature behavior and through high hydrolytic stability.
Suitable polyester glycols are preferably produced through esterification of an aliphatic and/or cycloaliphatic dicarboxylic acid with excess quantities of a diol. As preferred dicarboxylic acids are cited succinic acid, glutaric acid, adipic acid, pimelic acid, azelaic acid, and sebacic acid. The dicarboxylic acid is esterified with an excess of diol, preferably ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, 1,4-butane diol, 1,5-pentane diol and/or 1,6-hexane diol. Especially preferred is a polyester from adipic acid and ethylene glycol. At low temperatures polyester segments tend toward crystallization which impairs the elastic properties. A reduction of the tendency of the polyester chains to crystallize is preferably brought about through the incorporation of methyl branchings. This can take place through the partial replacement of the cited diols by other diols, such as 1,2-propane diol and 2,3-butane diol, or the use of methyl-substituted dicarboxylic acids. By using the cited longer-chain glycols, such as 1,4-butane diol, 1,5-pentane diol and/or 1,6 hexane diol, elastomers with increased hydrolytic stability are obtained. Suitable polyester glycols can also be obtained through the conversion of omega-hydroxy carboxylic acids with small quantities of diols, respectively through ring-opening polymerization of lactones with small quantities of diol. It is also possible to use mixtures of polyether glycols and polyester glycols. With respect to suitable macrodiols reference is also made to Ullmanns Enzyklopxc3xa4die der technischen Chemie, 3rd Edition, 1963, Publisher Urban and Scharzenberg, Miinchen Berlin, Vol. 14, pp. 344.
A polyurethane elastomer according to the invention comprises, in addition, a bivalent aliphatic, cydoaliphatic and/or aliphatic-cydoaliphatic moiety R2. It has surprisingly been found that the moiety R2 also determines significantly the melt behavior and thus the melt-spinnability, respectively melt-extrudability of the polyurethane elastomers. It was additionally found that through the selection of suitable moieties R2 the yellowing during the irradiation with high-energy radiation can be reduced, respectively eliminated. If the moieties R2 comprise aromatic moieties exclusively, the resulting polyurethane elastomers are no longer meltable in a nondecomposed state and thus are not melt-spinnable. In the presence of exdusively aromatic moieties R2 strong yellowing occurs during the secondary treatment with high-energy radiation. Therefore the polyurethane elastomers according to the invention comprise substantially nonaromatic moieties R2. However, in the polyurethane elastomers can be present aromatic moieties R2 up to such fraction that the desired properties of the elastomer are not impaired. Conventionally aromatic moieties are present at less than 20 mole percent, in particular less than 10 mole %, relative to the total quantity of moieties R2. The bivalent moiety R2 is preferably an alkylene group with 2 to 14 carbon atoms, a cycloalkylene group with 5 to 8 carbon atoms and/or an aliphatic-cycloaliphatic group with 7 to 24 carbon atoms. Especially preferred moieties R2 are traced back to a diisocyanate in the form of hexamethylene diisocyanate and/or dicyclohexylmethane-4,4xe2x80x2-diisocyanate.
The polyurethane elastomer according to the invention comprises further a short-chain olefinically unsaturated moiety X. The term xe2x80x9cshort chainxe2x80x9d indicates that the two Oxe2x80x94NHxe2x80x94, groups on both sides of the moiety X are at the most 14, in particular 11, bonds. removed from one another. By xe2x80x9colefinic unsaturationxe2x80x9d is understood that the moiety comprises one or several double or triple bonds capable of polymerization reactions. The double or triple bond can be in the main chain, however, it can also be disposed in a lateral side group.
The olefinically unsaturated moiety in one embodiment originates from a diaminoalkene, diaminoalkyne and/or diaminocycloalkene. In another embodiment the olefinically unsaturated moiety can be traced back to an alkene diol, alkyne diol, and/or cycloalkene diol. The amino, respectively hydroxy, groups can be located directly on carbon atoms from which the double or triple bonds originate. Such compounds comprise in general further substituents which stabilize the compound electronically. The amino or hydroxy groups can also be separated by one or several bonds from the carbon atoms from which the double or triple bond originates. Preferred examples of suitable diamines are cis- or trans-1,4-diaminobut-2-ene, cis- or trans4,4xe2x80x2-diarninostilbene, diamino maleic acid dinitrile, 1,4-diarminobut-2-ene and/or 3,6-diaminocydohexene-(1). Preferred examples of suitable diols are glycerine-1-allylether, cis- or trans-2-butene-1,4-diol, 2-butyne-1,4-diol and 5,6-bis-(hydroxymethyl)-bicyclo[2.2.1.]heptene-2. It is also possible to use mixtures of unsaturated diamines and/or diols with other amines, respectively diols, as long as the desired properties of the resulting polyurethane elastomer are not impaired.
The number n can be a number between 1 and 10. This represents the number of macrodiol moieties present within one soft segment under consideration. In an elastomer the number n is subject to fluctuations of statistical distribution. It is preferred that the average value of n is small, i.e. between 1 to 6, in particular 1 to 3.
The number of repeating soft segment/hard segment units m in a polyurethane elastomer chain is at least 4, preferably at least 8. Shorter chains cannot be melt-extruded, respectively melt-spun, due to the adhesiveness of the obtained products. m is conventionally in a range from 8 to 75.
Polyurethane elastomers according to the invention can comprise additives in the form of delustering means, color pigments, antioxidation agents, thermostabilizers, photo, respectively UW, stabilizers and/or hydrolysis stabilizers.
Subject matter of the invention is also a process for the production of the above denoted polyurethane elastomers. The polyether or polyester glycols to be used are preferably either already obtained moisture-free or, before the conversion through, for example, azeotropic conversion, are freed of adhering quantities of water. The macrodiol and a diisocyanate OCNxe2x80x94R2xe2x80x94NCO are converted at a molar ratio of approximately 1:5 to 1:1.1, in particular approximately 1:2 to 1:1.1, at a temperature between approximately 60 and 150xc2x0 C., preferably between approximately 80 and 135xc2x0 C. The conversion can be carried out without solvents in the melt or in a polar solvent, such as dimethyl formamide or dimethyl acetamide. Optionally a polyaddition catalyst, in particular dibutyltin dilaurate or dibutyltin diacetate, can be added to set a desired reaction level. With conversions in a solvent a catalyst is always required. Depending on the selected molar ratio, the macrodiols are xe2x80x9cpreextendedxe2x80x9d (NCO/OH less than 2) to form prepolymers via urethane or only linked at the chain ends with the diisocyanates (NCO/OH=2). The conversion is subject to the law of statistical distribution. The molar ratio of macrodiol to diisocyanate is between approximately 1:4 to 1:1.5. Typical molar ratios are approximately 1:4, approximately 1:3, approximately 1:2, approximately 2:3 and approximately 3:5.
The obtained prepolymers are subsequently converted in a second reaction step with an olefinically unsaturated diamine or diol as a chain extender. For this purpose the prepolymers are melted or dissolved in a suitable solvent, such as dimethyl formamide or dimethyl acetamide, and combined with the liquid, respectively melted, or with the chain extension means dissolved in the same or other solvent. The conversion preferably takes place at temperatures between approximately 70 and 150xc2x0 C., in particular at approximately 80 to 135xc2x0 C. Preferably a polyaddition catalyst, in particular dibutyltin dilaurate or dibutyltin diacetate is used. The melted or dissolved prepolymer is preferably added to the diamine or diol in order to avoid undesirable side reactions of excess isocyanate groups with already formed urea or urethane groups with the formation of cross linkages. To achieve a chain length of maximum size it is moreover desirable that the prepolymer and the chain extender be converted in such quantitative ratio that stoichiometric quantities of isocyanate functions and amine or hydroxy functions react with one another. The required quantity of chain extenders can be calculated from the originally used molar ratio of diisocyanate and macrodiol. Those quantities of diisocyanate are not taken into consideration, which are lost through impurities or traces of moisture. It is therefore preferred to determined the isocyanate group content of the prepolymer, for example, through titration and to calculate therefrom the required quantity of chain extenders.
In particular when using unsaturated diol chain extenders it is preferred to carry out the prepolymer synthesis as well as also the chain extension substantially in the absence of solvents.
In individual cases circumventing the prepolymer stage is also possible (xe2x80x9cone shot-processxe2x80x9d). Herein the diisocyanate reacts simultaneously with the macrodiol and the chain extenders. The reaction can take place in the melt as well as also in a suitable solvent.
Polyurethane elastomers according to the invention can be processed through conventional shaping or spinning methods, preferably through melt-spinning into a fiber, respectively through melt-extrusion into a film. Fibers produced in this way exhibit advantageous elastic properties, in particular favorable values of tensile strength, elongation at tear, residual elongation and heat distortion temperature (HDT).
To further improve the mechanical textile properties of the films, respectively threads, formed from the polyurethane elastomers according to the invention the covalent cross linkage of the double or triple bonds incorporated into the polymer chains is induced. To this end, the formed films, respectively threads, are exposed to high-energy radiation. The fibers, respectively the film, is treated with electron beams or UV rays. This secondary treatment leads to a marked improvement of the values for tensile strength, elongation at tear, residual elongation and heat distortion temperature of the fiber. A qualitative demonstration of the cross linkage is the extensive insolubility in solvents, such as dimethylacetamide of the irradiated fibers. While nonirradiated fibers are already after a short time dissolved in dimethylacetamide, the electron-irradiated fibers remain largely insoluble with the retention of their elastic properties.
The invention will now be explained in further detail in conjunction with embodiment examples and the enclosed FIGS. 1 to 6.